1. Field of the Invention
This invention relates to optical fiber communication systems. More particularly, this invention relates to an improvement in bidirectional optical fiber transmission systems.
2. Description of the Prior Art
Optical fiber transmission systems, which have many remarkable advantages over conventional transmission systems, are expected to prove useful in a wide range of applications from long-distance, large-capacity to short-range, small-capacity transmission. Efforts are being made across the world to develop such systems of superior performance.
One such bidirectional optical fiber transmission system which has been proposed uses a single strand of optical fiber with a pair of transmitter-receiver sets, primarily comprising a light emitting diode and a photodiode, respectively, coupled to each end of the fiber through an optical directional 3 dB coupler. This system is capable of full two-way communication employing light of the same emission spectrum (See, "Full Duplex Transmission Link over Single-strand Optical Fiber", by B. S. Kawasaki et al., at 1 Optics Letters at 107-08 (1977), and "Two-way Transmission Experiments over a Single Optical Fiber at the Same Wavelength Using Micro-optic 3 dB Couplers", by K. Minemura et al, at 14 Electronics Letters, at 340-42 (1978). This system which uses the same optical wavelength, is attractive both with respect to reliability and producibility of the optical devices compared with those which use different optical wavelengths.
However, as pointed out in both of the above-identified literature articles, part of the optical signal transmitted from the light emitting diode of one transmitter-receiver set to the other set reaches the photodiode of the same set, owing to internal reflections in and leakage from such optical elements as optical connectors or couplers, reflections from joints between the optical fiber and optical elements, and reflections from the splices of the optical fiber itself. There results a limitation in either the sensitivity of light received or the transmission distance. Such limitations cause design, construction, and implementation to be quite difficult.